The EUCAST Disk Diffusion Method for Antimicrobial Susceptibility Testing of Oral Anaerobes

Area of Science:

• Microbiology
• Clinical Diagnostics
• Infectious Diseases

Background:

• Standardized antimicrobial susceptibility testing (AST) methods are crucial for managing oral and extra-oral infections caused by anaerobic bacteria.
• Current methods may not adequately address the specific growth characteristics of various oral anaerobes.

Purpose of the Study:

• To evaluate the applicability of the recently published EUCAST disk diffusion method for rapidly growing anaerobes to common oral anaerobic bacteria.
• To determine optimal incubation times for accurate AST of Prevotella spp., Porphyromonas gingivalis, and Fusobacterium nucleatum complex.

Main Methods:

• Antimicrobial susceptibility testing was performed on 38 oral anaerobic strains (Prevotella spp., P. gingivalis, F. nucleatum complex) using amoxicillin and metronidazole disks.
• Disk diffusion plates were incubated anaerobically according to EUCAST guidelines, with inhibition zones assessed at 20 hours and 44 hours.

Main Results:

• The recommended agar supported growth for all tested strains.
• Twenty-hour incubation was sufficient for Fusobacterium strains.
• Prevotella species showed inconsistent growth within 20 hours, and all P. gingivalis strains required 44 hours for reliable zone assessment.

Conclusions:

• The EUCAST disk diffusion method is applicable to Fusobacterium nucleatum complex with standard incubation.
• Extended incubation (44 hours) is necessary for accurate AST of P. gingivalis and some Prevotella species.
• Further validation is required for prolonged incubation protocols for slow-growing anaerobes.